In addition to what OMD already said: some use dyneema, I don't see the point for UK size fish. My Rob Allen railgun came equipped with 2mm Rob Allen Mako line, which I think is rated for 400lb - again probably OTT for the UK but it lasts ages and is still very strong even when nicked/cut. It also means you are ready for a big fish if it shows up or you travel abroad.
If you can't find thick enough line, the long established mail order angling store Veals.co.uk in Bristol sometimes carry "big game" lines thick enough for the job - and sometimes they don't. However, probably easier to contact foxfish (
http://forums.deeperblue.com/goods-sale/78063-spear-gun-rigging.html) - that's what I have on my RA currently - or use my favourite back-up, cheap & cheerful
garden strimmer line. I've tried 2.5mm, 2mm but would suggest 1.65mm if you plan to try it (1.5mm looks too thin to me). It probably doesn't have the strength consistency of fishing mono, so leave a margin for error.
i agree, do u know anywhere in the uk
Re. UK suppliers. There seem to be only 3 or 4 specialist spearing suppliers plus several independent SCUBA stores: European Spearfishing Supplies (mainly Rob Allen gear), Spearo.co.uk (limited selection of carefully selected products) & apnea.co.uk (a retail & webstore with fairly extensive range of gear from Omer, Beuchat, Imersion and others). I think Portland Oceaneering may have closed(??? Cressi/Picasso) but there is a dive store in Paignton that seems to be getting into spearfishing gear (inc. Seacsub) - Venture Sports. Of course, it can be fairly easy to buy abroad these days too.
For smaller orders, perhaps a fully staffed retail store like apnea.co.uk in Jersey will be more likely to provide the level of service that you are looking for, and there is usually no VAT on small orders. I found apnea very helpful, knowledgeable and pleasant to deal with. I think spearo & ESS are primarily staffed by their owners, who have other jobs and also compete (& possibly guide) internationally - which no doubt complicates things.